Methods based on soil profile evaluation

The systems outlined by soil survey organizations such as the United States (Soil Survey Staff, 2010), Australia (Raymond, 2002), Canada (Day, 1983), United Nations (FAO, 2006), and developed for soil survey applications, have been evaluated for describing and interpreting soil structure. They require description of the morphology of a modal pedon, which generally implies a large soil pit or trench of 1.5 to 2 m deep. They could thus be used as well to assess the physical condition of the soil surface when the evaluation is focused on the surface as a seedbed for crops (McKeague et al., 1986).

Recently, three methods based on soil survey description have been developed to assess soil structure in agricultural soils, viz. the ‘Whole Profile Assessment’, the ‘Soilpak Scoring Procedure’, and ‘Le Profil Cultural’ (Table 1-1). These methods focus on detecting soil physical evidence of degradation processes as product of soil management.

The ‘Whole Profile Assessment’ developed by Batey (2000) is a soil profile description and land evaluation technique to assess land capability and particularly for the diagnosis of crop problems related to soil physical quality. This method was developed to be used in soils under any land use, to detect slight changes in physical conditions, to evaluate the capacity of the soil and to assess the potential limiting layers for plant growth (Boizard et al., 2005).

Other methods like the ‘SOILpak Scoring Procedure’ by McKenzie (2001) or ‘Le Profil Cultural’ by Roger-Estrade et al. (2004) are more advanced and provide detailed information on the complete soil profile (Mueller et al., 2009). The ‘SOILpak Scoring Procedure’ was designed originally to assess compaction under irrigated cotton on Vertisols. However, a revised ‘SOILpak Scoring Procedure’ for assessing soil ‘structural form’ was developed to be used on a wide range of soils for root growth relevance.

(McKenzie, 2001). It was shown to be flexible and sensitive with a wide range of criteria (Boizard et al., 2005).

‘Le Profil Cultural’ estimates the effects of cultural operations on soil structure and plant growth. The method is useful for analysing spatial and temporal variation in aggregate shape and porosity. It also comprises vertical and lateral stratification of the soil structure, which enable identification of the location of very highly compacted areas and the internal structural states of the soil profile. The method has been reported as a tool that allows directly linking the cultivation operations with soil structure dynamics, considering the spatial variation of soil structure as an element of the interpretation.

However, the method is time consuming and requires a high degree of scientific knowledge (Roger-Estrade et al., 2004).

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11 Table 1-1 Methods of field assessment of soil structure based on profile evaluation (Adapted from Boizard et al., 2005).

Method, author

and country Objective Key criteria Basis Scale of scoring

Whole profile assessment by Batey (2000)

Scotland

To evaluate the inherent capability of the soil to determine its potential for cropping To identify any limitations of crop growth as a result of soil management

To evaluate the soil potential for cropping:

texture, colour and potential rooting depth.

For the assessment of potential limiting layers: colour, development, strength and stability of structure; dense and compacted soil and degree of fissuring; the formation of saturated zones, anaerobic zones and the pattern of roots.

Description Soil structure score system based on Peerlkamp (1959)

To assess compaction under wide range of soil types and cropping systems

Soil structural shape, structural stability in water and structural resilience: crop root growth.

To conduct field studies on water transfer modelling and on denitrification

Transition between the tilled layers;

internal structural state of clods or zones;

type of structural state

Description Qualitative assessment:

areas with severe compaction;

or areas without any change in

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12 1.5.2.2. Methods based on topsoil examination

Methods related to topsoil examination are focused on the top 20 to 30 cm of soil and describe the status or condition of a specific soil due to relatively recent land use or management decisions. Several topsoil examination methods have been proposed (Table 1-2), differing in aspects such as depth of the soil under consideration, handling the soil prior to assessment, emphasis placed on particular features of soil structure, and Peerlkamp (1959). This method was developed to assess the soil physical characteristics in a systematic way by numerically assessing the soil structure quality within the topsoil or tilled layer (Boizard, et al., 2005). Some modifications to the Peerlkamp method have been proposed and have been compared with the original (Batey, 2000; Ball and Douglas, 2003; Ball et al., 2007).

Ball et al. (2007) revised and updated the Peerlkamp method, and proposed the Visual Evaluation of Soil Structure (VESS). The proposed method involves the use of a visual key with well-defined descriptions of criteria for each category. VESS includes soil layering, which constitutes the main improvement from the original method. However, according to Mueller (2009), modifications of the Peerlkamp method are very fast in handling but prone to subjective scorings. If one or more features are not present for this description, the operator can underestimate the structural quality.

Based on the previous modifications proposed by Ball et al. (2007), Guimarães et al. (2011) developed improvements on VESS. Although some researchers suggest subjectivity in the way of breaking up the soil block, they showed that breaking up a soil block by hand or by dropping results in the same soil quality score. They also found that reducing large aggregates to 1.5-2.0 cm fragments and describing their shape and porosity, helps to identify visual score particularly in the middle range of soil quality.

Furthermore, modifications proposed to this technique, tested in soils from Scotland and Brazil, suggest that the modified version is a more practical and objective evaluation of soil structural quality compared to the original one (Guimarães et al., 2011).

Other methods including fertility determinant aspects such as root growth, organic residues or fauna within individual layers in the topsoil are those of Beste (1999) and Munkholm (2000). Beste´s system is not as detailed as Munkholm´s but it is extended with the quantitative determination of some soil physical properties. Beste´s system originates from the method of ‘The Spade Diagnosis’ developed by J. Görbing in about 1930. According to Beste (1999), this method is ‘based on farmers’ knowledge and combines the actual comprehensive and qualitative impression of soil condition in the field with exact and quantitative data information about soil parameters from same location’.

Chapter 1

13 Table 1-2 Methods of field assessment of soil structure based on topsoil examination (Adapted from Boizard et al., 2005)

Method, author

and country Objective Key criteria Basis Scale of scoring

Visual method of soil structure evaluation by Peerlkamp (1959) the Netherlands

To assess the soil physical characteristics systematically in terms of numeric assessment of the soil structure quality within the topsoil or tilled layer

Size and shape of aggregates, cohesion of soil particles, porosity, root development, dispersion of the soil surface feature, soil colour, earthworms, potential rooting depth, soil surface layer regarding soil structure and rooting characteristics and relate to past management practices

Ground cover; soil layering; moisture;

texture; structural elements, macropores, root growth, soil fauna, decomposition of

To incorporate simplified structural descriptions into a scale of structural quality

Size, shape and strength of aggregates, porosity, colour and roots

Guideline photographs

1= good, 5= poor

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14 The Soil Quality Scoring Procedure (SQSP) developed by Ball and Douglas (2003) is presented as a semi-quantitative visual and tactile method for assessing soil physical fertility in terms of soil structure, root growth and soil surface condition based on Munkholm (2000) and ranks soils according to a marking scheme similar to that of Beste (1999). According to Ball and Douglas (2003), the SQSP has as advantage that a brief, standardized description of the soil is provided which is summarized into three ranks of soil condition. The method implies layering of the soil sample evaluated and scores for structure and rooting conditions are weighted to the thickest apparent layer.

For New Zealand soils, Shepherd (2000) developed the Visual Soil Assessment (VSA) method, based on the visual assessment of key soil state and plant indicators of soil quality, presented on a score card. Each indicator used by VSA is considered as a separate entity. In this way each indicator is a useful early warning of changes in soil conditions. For an overall score of the soil condition each indicator is weighted by a factor of importance.

VSA is considered to provide a valid semi-quantitative assessment of soil quality, in terms of the criteria defined, and it can therefore be used in conjunction with, and complement, quantitative laboratory measurements (Shepherd and Park, 2003; Shepherd, 2009).

Due to many early and new methods developed to assess soil structure directly in the field, the Working Group F ‘Visual Soil Examination and Evaluation’ of the International Soil & Tillage Research Organisation (ISTRO) promoted a field meeting in northern France during which several methods were compared, including those mentioned above (Boizard et al., 2005). The results of each test were presented to the whole group, which was able to question and discuss the findings. They concluded that:

(i) ‘Each method has been developed to answer a specific question in a specific locality’,

(ii) ‘Any transfer of techniques from one area to another must be done with care and sensitivity’, and

(iii) ‘The selection of one or different methods to assess soil structure depends on why and who will perform the test’.

Until now, field assessment methods have been tested and compared by researchers in pasture and crop areas of some ‘temperate’ and ‘subtropical’ soils, but the evaluation and comparison of these methods under ‘tropical’ soils is still missing. In this context ‘tropical soil’ refers to all those soils geographically located within the Tropic of Capricorn and Tropic of Cancer without any distinction in evolution state. Similarly, evaluation of the suitability of these methods in soils under different land uses or in soils under different conditions in a more comprehensive approach with respect to other soil physical indicators is also requested. In part II of this dissertation, Chapters 6, 7 and 8, the use of visual examination methods for assessing soil structural quality is addressed.

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15 1.6. Linking and interpreting soil structural quality using an integrated framework Soil properties that have been used as the most important indicators for evaluating soil physical quality are bulk density (BD), porosity, air capacity (AC), field capacity (FC), plant available water capacity (PAWC), soil organic matter (SOM), hydraulic conductivity, aggregate stability and penetration resistance (Karlen et al., 1998, Reynolds et al., 2002, 2007; Shukla et al., 2006; Osman, 2013). They provide direct quantitative estimations of the ability of a soil to store and transmit root zone water and grow crops.

Internationally, there are several indices of soil physical quality, soil quality kits and frameworks that comprise different indicators (Karlen et al., 2003), most of them including those mentioned above. But not many include structural form description as indicators. This leads to a separated indirect and direct evaluation of the structural quality, which does not follow the ideal evaluation of the soil structure (structural form, aggregate stability and resilience). However, Carter (2004) states that research focussed on structural complexity in agricultural soils are ‘ongoing to provide an improved understanding of soil structure and structure mediated processes and to develop or modify appropriate soil structure methodology’.

This point is sustained by the work of Pachepsky and Rawls (2003) who found that qualitative morphological observations of soil could be translated into quantitative soil parameters and used in an integrated framework. Many others have tested the inclusion of structural form indicators as potential independent variables for predicting other structure-related properties (Lilly et al., 2008; Vereecken et al., 2010; Nguyen et al., 2014).

The use of a data-driven statistical technique for integrating quantitative and qualitative evaluations of soil structure is the main focus of Chapter 9 of this dissertation.

On the other hand, the interest in developing ‘unique’ indicators for soil structure and soil quality assessment, which is a simplistic approach, can be found in the literature as well. Examples include indices of soil structure based on soil characteristics related to this property such as particle size distribution and soil organic matter (SOM) such as the instability index (Henin et al., 1958), the index of crusting (FAO, 1980), and the structural stability index (Pieri, 1992). These indices involve SOM content per se, from which in some cases, differences in soil classifications according to their structure stability or quality become evident.

Generally, SOM promotes aggregate stability because it reduces aggregate swelling, and increases the intrinsic strength of aggregates (Fortun and Fortun, 1989).The effectiveness of SOC forming stable aggregates is related to its decomposition rate, which in turn is influenced by physical and chemical protection to microbial action (Bronick and Lal, 2005). However, inconsistencies in proportional relationship between SOM content and structural quality (mainly aggregate stability) has also been mentioned. Therefore, aggregate stability may depend more on the type of SOM and its provisions relating to the mineral particles (Fortun and Fortun, 1989; Holeplass et al., 2004). The SOM constituents link the primary particles in aggregates physically and chemically (Lado et al., 2004).

Therefore, content and distribution of the stable and unstable aggregates in soil have

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16 close association with SOM dynamics and soil quality, hence, the soil degradation problems could be evaluated studying the proportion of stable aggregates (Márquez et al., 2004). It is however, important to emphasize that many other factors, apart from SOM, are related to structural stability. Recently, it has been suggested that 2:1 clay minerals contribute to the formation and stabilization of different aggregate-size classes differently (Fernández‐Ugalde et al., 2013).

Another example of simplistic approaches is the soil physical quality index S (Dexter, 2004a), which is the slope of the soil water release curve (SWRC) at its inflection point. Although this index was developed based on the idea of integrating observations of a range of soil properties to obtain an overall assessment, it only represents a particular value of the SWRC. S index has been criticised as providing inconsistent designations of soil physical quality and lacking consistency with other physical indicators for some soils (Reynolds et al. 2009; Van Lier, 2014). When the objective is to use an indicator to predict specific soil property or soil function, then the existence of the complexity of the soil structure is neglected. The use of simpler approaches is discussed in Chapter 10 of this dissertation.

Despite the numerous methods for characterizing soil structure, none of these have been accepted universally. In each case, the choice of the method to be used depends on the problem, the soil and the equipment available (Hillel, 1998), but also on the scale and scope of the study.

According to Eswaran et al. (2001), ‘soil scientists have an obligation not only to show the spatial distribution of stressed systems but also to provide reasonable estimates of their rates of degradation’. This justifies the efforts of several researchers in their emphasis for selecting the most suitable warning indicators of soil degradation. Indicators of soil degradation or soil quality judiciously selected can play a significant role in assisting national decision-makers to develop appropriate land use and conservation policies.